I wonder if the recent huge Toyota engine sludge lawsuit has something to do with this. Mercedes Benz has a similar problem and has also extended their warranties on certain engines. The articles I've seen say that if you've used Synthetic oil, this isn't a problem. However, if you use dino oil, this can be a problem.
Not all engines produce sludge. In the case of certain Toyota and Mopar V-6's the culprit was a hyperactive PCV system. Less sludge with syn but you'd still get it. The best defense there is simply to buy the cheapest generic oil and filters you can find and change it every 2000 miles.